A gas turbine engine generally includes a fan and a core arranged in flow communication with one another with the core disposed downstream of the fan in the direction of the flow through the gas turbine. The core of the gas turbine engine generally includes, in serial flow order, a compressor section, a combustion section, a turbine section, and an exhaust section. With multi-shaft gas turbine engines, the compressor section can include a high pressure compressor (HP compressor) disposed downstream of a low pressure compressor (LP compressor), and the turbine section can similarly include a low pressure turbine (LP turbine) disposed downstream of a high pressure turbine (HP turbine). With such a configuration, the HP compressor is coupled with the HP turbine via a high pressure shaft (HP shaft), and the LP compressor is coupled with the LP turbine via a low pressure shaft (LP shaft).
In operation, at least a portion of air over the fan is provided to an inlet of the core. Such portion of the air is progressively compressed by the LP compressor and then by the HP compressor until the compressed air reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section through the HP turbine and then through the LP turbine. The flow of combustion gasses through the turbine section drives the HP turbine and the LP turbine, each of which in turn drives a respective one of the HP compressor and the LP compressor via the HP shaft and the LP shaft. The combustion gases are then routed through the exhaust section, e.g., to atmosphere.
The LP turbine drives the LP shaft, which drives the LP compressor. In addition to driving the LP compressor, the LP shaft can drive the fan through a power gearbox of an epicyclic gearing arrangement, which allows the fan to be rotated at fewer revolutions per unit of time than the rotational speed of the LP shaft for greater efficiency. The power gearbox rotatably supports a sun gear that is disposed centrally with respect to a ring gear and a plurality of planet gears, which are disposed around the sun gear and engage between the sun gear and the ring gear. The LP shaft provides the input to the epicyclic gearing arrangement by being coupled to the sun gear, while the fan can be coupled to rotate in unison with the carrier of the planet gears or with the ring gear. Each planet gear meshes with the sun gear and with the ring gear. One of the carrier or the ring gear may be held stationary, but not both of them. Each planet gear is rotatable on its own bearing that is mounted on a support pin housed within the power gearbox, which is fixed to the peripheral region of the carrier of the epicyclic gearing arrangement. The shaft of the fan is rotatable on its own bearing that is housed in a sun gearbox, which is also called the fan gearbox.
For any given gas turbine engine application, the planet gears are designed to provide a set reduction ratio between the rotational speed of the LP shaft and the rotational speed of the fan shaft. Because each power gearbox that houses each planet gear is disposed within the flow path of the gas turbine engine, the challenge is to design on the one hand a reliable and robust power gearbox that meets all flight conditions of the engine while on the other hand designing a power gearbox that is compact sufficiently to fit inside the flow path in a way that does not require the entire engine size to be larger and heavier than otherwise would be needed in order to accommodate the power gearbox.
The carrier for the planet gears of the power gearbox desirably is formed as a single monolithic part so as to minimize gear misalignment. However, this one piece carrier can complicate mounting each planet bearing to the carrier. Mounting each planet bearing to the carrier via a conventional support pin that is held in the carrier by a bolt and spanner nut configuration involves the added weight of the support pin and the spanner nut. In order to meet the necessary design requirements, the clamp loads from the support pin and spanner nut configuration result in very high axial loads. These increased loads reduce design robustness and add weight to the design. The highest stresses in the existing support pin design are believed to result from the stresses induced by the spanner nut torque that must be applied during the mounting assembly rather than from stresses that occur during normal operation of the power gearbox.
Moreover, because the current support pin design requires a press fit along a substantial length of the support pin and inner ring of the bearing, the following assembly problems are presented. More than a six inch long press fit support pin must be dropped into two sides of the carrier, and this requires a large temperature difference between the support pin and the inner ring and two sides of the carrier. This creates an assembly risk whereby the support pin fails to drop all the way through the three mating pieces, and thus once the temperatures normalize the support pin will become stuck without the threads exposed. It then will become necessary to remove the support pin from the assembly, and a second attempt at assembly must be performed. The loads needed to remove the stuck support pin are high, and the failed attempt presents the additional risk that the surrounding hardware or the support pin itself becomes damaged in the process of removal and attempted reinsertion.